This invention relates to an exposure apparatus for use in fabricating of printed circuit boards and so on.
A method for forming a predetermined electric circuit pattern onto a substrate for a printed circuit board is known, in which masking is applied to requisite portions of a copper layer on the substrate, unrequisite portions are removed by means of etching and, then, the masking layer is removed.
The masking has conventionally been carried out in the following manner: a mask film having recorded thereon a predetermined pattern is brought into close contact with a photosensitive layer, such as a photoresist provided on the substrate for the printed circuit board. The photosensitive layer is then exposed to intensive ultraviolet rays. Subsequently, the exposed photosensitive layer is developed. According to this masking method, the development makes it possible to form the mask layer in conformity with the pattern on the mask film.
FIGS. 1 and 2 are views for explaining the exposure processings according to the above-described conventional exposure method.
In the figures, reference numeral 1 denotes a substrate for a printed circuit board. The substrate 1 is provided on both sides thereof with respective photosensitive layers. Reference numerals 2 and 3 denote, respectively, upper and lower mask films. Negative or positive films are employed as the mask films 2 and 3, depending upon the properties of the photosensitive material.
After positioning the substrate 1 and the upper and lower mask films 2 and 3, an encaustic frame 5 having stretched thereon a Mylar film 4, is downwardly moved, as viewed in FIG. 1. The encaustic frame 5 is elastically urged against a rubber sealing member 7 mounted on an encaustic glass frame 6, so that the substrate 1 is isolated from the environmental air. A space between the Mylar film 4 and the encaustic glass frame 6 is then vacuum-evacuated to bring the substrate 1 and the upper and lower mask films 2 and 3 into close contact with each other, as shown in FIG. 2.
Subsequently, the ultraviolet rays are irradiated as indicated by the arrows in FIG. 2, to expose the photosensitive layers to the ultraviolet rays, thereby photo-sensitizing the photosensitive layers in accordance with the patterns on the respective upper and lower mask films 2 and 3.
However, several problems exist when the above-described conventional exposure method using the mask films are used.
First, once the mask films 2 and 3 and the substrate 1 have been positioned, it is no longer possible to move the mask films 2 and 3 and the substrate 1 relative to each other, because they are in close contact with each other. Accordingly, the positioning can only be carried out under such a condition that the mask films 2 and 3 and the substrate 1 slightly float relative to each other. By this reason, it is difficult to insure the identity between the initially set relative positional relationship among the mask films 2 and 3 and the substrate 1 and the relative positional relationship at the time they are brought under vacuum into close contact with each other. This is not preferable in view of the desired accuracy.
Secondly, direct contact between the mask films 2 and 3 and the substrate 1 sometimes causes flaws on the mask films 2 and 3, resulting in a reduction of the pattern accuracy.
Thirdly, since the mask films 2 and 3 are brought into close contact with the substrate 1, a time is necessary for the vacuum evacuation in addition to the period of time for the exposure which is the primary object. This hinders an improvement in the throughput, i.e., the processing efficiency.
Moreover, it is necessary to replace the mask films 2 and 3 with another one each time the circuit pattern to be formed on the substrate 1 changes. Thus, the cost of manufacture increases, particularly in the case of a multikind small-quantity production.